1. Field of the Invention
The present invention relates to the manufacture of mortars and concretes from hydraulic binders, and, more especially, relates to the manufacture of mortars and concretes which are particularly well adapted for more rapid setting and hardening, in particular in cold weather, so that same quickly attain high mechanical strengths.
2. Description of the Prior Art
It is well known to this art that mortars and concretes are prepared by mixing hydraulic binders with water and aggregates such as sand, gravel or pebbles.
The expression "hydraulic binders" denotes ingredients such as, for example, cement and line, which are capable of setting by the interaction of water and the constituents of the said binders. It is known that the hydration of cements proceeds in accordance with a two-stage mechanism: the first, which takes a few hours, is referred to as "the setting" and the second, which takes place after the setting, is referred to as "hardening".
The fresh mortar or concrete prepared utilizing these cements is typically introduced into a form or mold defining the ultimate shape of the structure (or part of the structure) which it is desired to obtain. The setting and the hardening are therefore very important phenomena from an industrial point of view because, in order to be able to remove the form or strip it from the mortar or the concrete, it is necessary to wait, on the one hand, until the setting has taken place, and, on the other hand, until the hardening has advanced to a certain degree so as to obtain high initial compressive strengths on the order of at least 40 bars.
Normally, the users of cements desire to be able to obtain much initial strength and to remove the forms or frames within a reasonable time of not more than 24 hours.
Now, the hydration of the cement is a chemical conversion reaction which depends on the temperature. Since the temperatures are constantly varying, it is essential to have available effective means for controlling the rate of hydration for the manufacture of high-grade mortars and concretes. This is particularly important for the manufacture and the use of mortars and concretes during the winter when an accelerated hydration is required in order to avoid the damage caused by very low temperature and in order to obtain, within the desired time, high mechanical strengths which make it possible to strip therefrom the forms.
It is possible to achieve a quick hydration by external heating; but this is an expensive technique because of the equipment required and the heating process itself.
Another means consists in adding an accelerator to the mixture constituting the mortar or the concrete. Calcium chloride is very frequently mentioned among the known accelerators; however, although this compound is very effective for accelerating the setting and the hardening of cements, it is only reluctantly used in the case of reinforced concretes and prestressed concretes because it has proved very corrosive towards the iron bars which comprise the reinforcement for said concretes.
It is also known to manufacture mortars and concretes, which are capable of providing improved initial mechanical strengths, by adding a water-reducing agent to the mixture used for their preparation. The expression "water-reducing agent" denotes a dispersant which makes it possible, given the same degree of workability, to reduce the water content of a given concrete or which, for the same water content, substantially increases this workability, or, alternatively, which makes it possible to achieve these two effects simultaneously.
It is generally considered that the amount of water which is stoichiometrically required in order to cause the cement to harden is on the order of 30% by weight, relative to the cement (the weight ratio water/cement, subsequently referred to as the ratio W/C, is 0.3). However, the cement is only fluid and can only be readily worked by hand if water is added in a proportion on the order of about 50% by weight (W/C=0.5). When a dispersant/water-reducing agent is employed, it is then possible to prepare a paste having the same good fluidity by adding water in proportions of less than 50% by weight.
This results in more rapid setting and hardening, which are due to the fact that the amount of water approaches the amount which is stoichiometrically required for the hydration of the hydraulic binder. However, it has been found that this means is still not perfect; in fact, it has been observed that the acceleration in the setting and hardening which it effects does not make it passible to botain, after 24 hours, at temperatures as low as, for example, 0.degree. C. to 10.degree. C., compressive strengths which are compatible with the stripping of the forms.
Finally, there has previously been no technique which is capable of completely satisfying the need, experienced by the users of hydraulic binders of making available a process which provides mortars and concretes capable of setting and hardening more rapidly, especially in cold weather, in order to make it possible to strip the forms therefrom with the reasonable time desired.